Patients who've tried to charge their medical marijuana purchases to American Express recently made an unpleasant discovery: As of April 30, American Express started denying all such transactions. "American Express has made a decision not to allow card acceptance for medical marijuana," says company spokesperson Diana Postemsky. "It's our policy to adhere to federal law in such matters."

By adhering to federal law, though, American Express may have left numerous medical marijuana patients who've previously used AmEx for such purchases in the lurch -- not to mention all the medical marijuana centers and peripheral businesses that have had to deal with significant red tape just to get credit-card machines into their stores. And the fact that American Express is willing to turn its back on a rapidly growing industry could signal that it might not be the only credit-card company to start rejecting medical-marijuana buys.

While Representative Jared Polis is introducing legislation to help banks feel confident that they can do business with medical marijuana operations without fear of federal reprisals, there's no guarantee that measure will make American Express change its position. "Our bill is basically trying to take away the threat of banks being investigated and having to report information so that they will do business with fully state-certified and legal medical marijuana businesses," says Chris Fitzgerald, Polis's communications director. "So AmEx's decision may or may not be impacted here. AmEx can basically decide if they want to do business with an establishment, and whether that establishment can accept AmEx."

So contrary to what those American Express commercials used to advise, medical marijuana patients who once relied on the card should now feel free to leave home without it.

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Joel Warner is a former staff writer for Westword and International Business Times. He's also written for WIRED, Men's Journal, Men's Health, Bloomberg Businessweek, Popular Science, Slate, Grantland and many other publications. He's co-author of the 2014 book The Humor Code: A Global Search for What Makes Things Funny, published by Simon & Schuster.